The heart is a muscular organ comprising multiple chambers that operate in concert to circulate blood throughout the body's circulatory system. As shown in FIG. 1, the heart 100 includes a right-side portion or pump 102 and a left-side portion or pump 104. The right-side portion 102 includes a right atrium 106 and a right ventricle 108. Similarly, the left-side portion 104 includes a left atrium 110 and a left ventricle 112. Oxygen-depleted blood returning to the heart 100 from the body collects in the right atrium 106. When the right atrium 106 fills, the oxygen-depleted blood passes into the right ventricle 108 where it can be pumped to the lungs (not shown) via the pulmonary arteries 117. Within the lungs, waste products (e.g., carbon dioxide) are removed from the blood and expelled from the body and oxygen is transferred to the blood. Oxygen-rich blood returning to the heart 100 from the lungs via the pulmonary veins (not shown) collects in the left atrium 110. The circuit between the right-side portion 102, the lungs, and the left atrium 110 is generally referred to as the pulmonary circulation. When the left atrium 110 fills, the oxygen-rich blood passes into the left ventricle 112 where it can be pumped throughout the entire body. In so doing, the heart 100 is able to supply oxygen to the body and facilitate the removal of waste products from the body.
To circulate blood throughout the body's circulatory system as described above, a beating heart performs a cardiac cycle that includes a systolic phase and a diastolic phase. During the systolic phase (e.g., systole), the ventricular muscle cells of the right and left ventricles 108, 112 contract to pump blood through the pulmonary circulation and throughout the body, respectively. Conversely, during the diastolic phase (e.g., diastole), the ventricular muscle cells of the right and left ventricles 108, 112 relax, during which the right and left atriums 106, 110 contract to force blood into the right and left ventricles 108, 112, respectively. Typically, the cardiac cycle occurs at a frequency between 60 and 100 cycles per minute and can vary depending on physical exertion and/or emotional stimuli, such as, pain or anger.
The contractions of the muscular walls of each chamber of the heart 100 are controlled by a complex conduction system that propagates electrical signals to the heart muscle tissue to effectuate the atrial and ventricular contractions necessary to circulate the blood. As shown in FIG. 2, the complex conduction system includes an atrial node 120 (e.g., the sinoatrial node) and a ventricular node 122 (e.g., the atrioventricular node). The sinoatrial node 120 initiates an electrical impulse that spreads through the muscle tissues of the right and left atriums 106, 110 and the atrioventricular node 122. As a result, the right and left atriums 106, 110 contract to pump blood into the right and left ventricles 108, 112 as discussed above. At the atrioventricular node 122, the electrical signal is momentarily delayed before propagating through the right and left ventricles 108, 112. Within the right and left ventricles 108, 112, the conduction system includes right and left bundle branches 126, 128 that extend from the atrioventricular node 122 via the Bundle of His 124. The electrical impulse spreads through the muscle tissues of the right and left ventricles 108, 112 via the right and left bundle branches 126, 128, respectively. As a result, the right and left ventricles 108, 112 contract to pump blood throughout the body as discussed above.
Normally, the muscular walls of each chamber of the heart 100 contract synchronously in a precise sequence to efficiently circulate the blood as described above. In particular, both the right and left atriums 106, 110 contract (e.g., atrial contractions) and relax synchronously. Shortly after the atrial contractions, both the right and left ventricles 108, 112 contract (e.g., ventricular contractions) and relax synchronously. Several disorders or arrhythmias of the heart can prevent the heart from operating normally, such as, blockage of the conduction system, heart disease (e.g., coronary artery disease), abnormal heart valve function, or heart failure.
Blockage in the conduction system can cause a slight or severe delay in the electrical impulses propagating through the atrioventricular node 122, causing inadequate ventricular relaxations and filling. In situations where the blockage is in the ventricles (e.g., the right and left bundle branches 126, 128), the right and/or left ventricles 108, 112 can only be excited through slow muscle tissue conduction. As a result, the muscular walls of the affected ventricle (108 and/or 112) do not contract synchronously (e.g., asynchronous contraction), thereby, reducing the overall effectiveness of the heart 100 to pump oxygen-rich blood throughout the body. For example, asynchronous contraction of the left ventricular muscles can degrade the global contractility (e.g., the pumping power) of the left ventricle 112 which can be measured by the peak ventricular pressure change during systole (denoted as “LV+dp/dt”). A decrease in LV+dp/dt corresponds to a worsened pumping efficiency and less cardiac output.
Similarly, heart valve disorders (e.g., valve regurgitation or valve stenosis) can interfere with the heart's 100 ability to pump blood, thereby, reducing stroke volume (i.e., aortic pulse pressure) and/or cardiac output.
Various medical procedures have been developed to address these and other heart disorders. In particular, cardiac resynchronization therapy (“CRT”) can be used to improve the conduction pattern and sequence of mechanical contractions of the heart. CRT involves the use of an artificial electrical stimulator that is surgically implanted within the patient's body. Leads from the stimulator can be affixed at a desired location within the heart to effectuate synchronous atrial and/or ventricular contractions. Typically, the location of the leads (e.g., stimulation site) is selected based upon the severity and/or location of the blockage. Electrical stimulation signals can be delivered to resynchronize the heart, thereby, improving cardiac performance.
Despite these advantages, several shortcomings exist that limit the usefulness of CRT. For example, results from many clinical studies have shown that hemodynamic response to CRT typically varies from patient to patient, ranging from very positive (e.g., improvement) to substantially negative (e.g., deterioration). Thus, in order to predict the benefit from CRT, the patient typically must be screened prior to receiving the therapy. One common method that predicts hemodynamic response to CRT relies on measurement of a surface electrocardiogram (ECG). Such measurement is often performed manually and is subject to human error. Additionally, it is difficult to implement such a surface measurement with an implantable device thereby making it difficult to continuously monitor the response.
Thus, there is a need for improved methods and systems that can automatically and reliably predict whether a patient will have a positive response to CRT and/or be able to monitor the response continuously during the entire course of CRT.